But how likely is it? There are two key sources of uncertainty about how hot it is likely to get in the future. The first comes from the Earth’s system itself – scientists are unable to say precisely how sensitive the atmosphere is to elevated levels of greenhouse gases.Estimates for how much temperatures will rise following a doubling of carbon dioxide from pre-industrial levels vary from a survivable 1.9°C to a definitely-not-survivable 11.5°C, according to the first results from Oxford University’s ClimatePrediction experiment. The end-Permian mass extinction – the worst die-off of all time, when life came close to disappearing from the planet 250 million years ago – is now thought to have been caused by an deadly interval of global warming, which heated the oceans to such an extent that they became stagnant, killing up to 95 per cent of marine organisms and devastating terrestrial life too.A repeat of a catastrophe like this would certainly lead to the collapse of human civilisation and the deaths of billions of people. A landmark paper in Nature last year estimated this extinction risk, and concluded that up to a third of species would be “committed to extinction” by 2050 because of global warming.There are precedents for this kind of grim scenario: extreme greenhouse episodes in the distant geological past are clearly associated with major extinction pulses.

Retreating mountain glaciers often leave lakes behind, which can overflow catastrophically when hit by landslides or avalanches. Species can only survive in a particular climate envelope, and once temperatures shift beyond their adapted range, they must either move or die out. A glacial lake at the base of Annapurna II burst in 2003, killing five people in the ensuing flash flood.No part of the globe will remain untouched as global warming accelerates, pushing temperatures to heights not experienced on Earth for millions of years. Already scientists are defining the rise of the human species as the start of a new geological era, the “anthropocene”, which unfortunately looks set to be characterised by a rapidly-changing climate and mass extinctions throughout the natural world.Plant and animal behavior is already changing in response to rising temperatures – the virtual disappearance of sand eels from the North Sea ecosystem and resulting crash in seabird breeding numbers is a striking example. Nepal is already dotted with dangerous glacial lakes – the last count identified 26. Whilst this will cause water deficits in years to come, increased runoff due to the melting also holds dangers. According to the Switzerland-based World Glacier Monitoring Service, 96 per cent of glaciers surveyed – from Kazakhstan to Bolivia – are currently retreating.

In this case, hundreds of millions of people could be affected throughout Asia, from northern China to Vietnam.Glaciers are already disappearing from mountain ranges across the world. The Indus river in particular, on which the overwhelming majority of Pakistan’s food production depends, will lose much of its flow once the glaciers in the Karakoram recede. Crisis on an even larger scale will affect the Indian sub-continent once Himalayan glaciers become severely depleted by warming. In Peru, for example, the glaciers which keep desert cities like Lima alive are receding rapidly, and may be gone completely within as little as 20 or 30 years.Millions of people will have to find new supplies of water or become environmental refugees. Several scientific studies have identified the 2003 heatwave as a harbinger for the future – one paper published last year in the journal Global and Planetary Change predicted that summer heatwaves like 2003 – which killed an estimated 35,000 people across Europe – will be the norm by the latter part of this century.Global agriculture is also threatened by the loss of mountain glaciers, which supply water to arid lowland regions in South America and Asia.

Higher temperatures and falling water supplies are expected to lead to a reduction in overall harvests, even as new areas are opened up to farming in once-cold places like Canada and Siberia.The heatwave of 2003 reduced the grain harvest in continental Europe by 30 million tonnes, helping push world grain stocks to their lowest level in three decades. Areas of old stabilised sand dunes, covered by vegetation and currently supporting millions of subsistence farmers and animal herders, in a huge area from southern Zambia to Botswana and South Africa, could remobilise as windspeeds increase and rainfall totals diminish over coming decades.A similar process could also begin in the High Plains of the US, where major agricultural states such as Nebraska could also find themselves suddenly overrun with shifting sands.The implications for global food production can only be guessed at, but according to Lester Brown of the Earth Policy Institute, humanity will be experiencing net food deficit sooner than anyone currently expects. A paper published in the science journal Nature in June by Oxford University’s David Thomas and colleagues suggested that much of southern Africa will have turned to desert by the end of the century. Feats of travel, building and farming that were impossible before suddenly became easy with fossil fuel-powered technology.